Filters used to provide purified air to confined areas are referred to as collective protection (CP) filters. CP filters differ from respirator or cartridge filters in that CP filters are operated on a continuous or near continuous basis and are designed to protect personnel located in a confined area. Respirator filters are used in conjunction with a gas mask and are designed to be used only in the event of a chemical incident, or when a chemical threat is imminent. Because of the continuous or near continuous operation of the CP filter, unexpected deactivation of the filtration media is of concern.
Collective protection (CP) filters are designed to remove toxic chemicals, such as traditional chemical warfare (CW) agents and toxic industrial chemicals (TICs) from air, thereby providing safe breathing to personnel in a chemically contaminated environment. Collective protection filters are designed to treat large volumes of air entering rooms, shelters, buildings, vehicles, etc. The volumes of air treated by CP filters varies, e.g., from about 50 standard cubic feet per minute (SCFM) to greater than about 1,000 SCFM, and in some cases, greater than about 10,000 SCFM.
For purposes of this application, a toxic chemical is defined as any chemical present in the vapor phase that may cause harm to a human. CW agents and TICs are examples of toxic chemicals. Examples of traditional chemical warfare agents include hydrogen cyanide (HCN, also known as AC), chlorine gas (Cl2), phosgene (COCl2, also known as CG), cyanogen chloride (ClCN, also known as CK), mustard (bis(2-chloroethyl) sulfide, also known as HD), sarin ((RS)-Propan-2-yl methylphosphono-fluoridate, also known as GB) and O-ethyl S-[2-(diisopropylamino)ethyl] methylphosphonothioate (also known as VX). Further examples of traditional CW agents are provided in Army Field Manual 3-9 which is hereby incorporated herein by reference. Toxic Industrial Chemicals may be defined as chemicals with an LCt50 of less than about 100,000 mg-min/m3 and manufactured in quantities exceeding about 30 tons per year at a single facility. Examples of TICs include sulfur dioxide (SO2), hydrogen sulfide (H2S), chlorine gas (Cl2), fuming nitric acid (HNO3), nitrogen dioxide (NO2), formaldehyde (CH2O), ammonia (NH3) and mixtures of two or more of the same.
Current CP filters designed to remove CW agents and TICs from air may contain activated, impregnated carbon. ASZM-T is one example of activated, impregnated carbon which comprises activated carbon onto which salts of copper, zinc, molybdenum and silver are loaded, along with triethylene diamine (TEDA). When freshly prepared, ASZM-T has a high capacity for the removal of CW agents and selected TICs; however, ASZM-T degrades as a result of prolonged exposure to humid air, and degrades even faster when exposed to humid air containing low levels of airborne contaminants, such as, e.g., SOx, NOx and fuel vapors. Degradation of ASZM-T results in a decreased protection capability of the filter and costly filter change-outs.
Unlike individual protection filters (respirator cartridges), filters employed in collective protection applications are, as mentioned, in operation on a continuous or near continuous basis. As a result, these filters may be exposed to large volumes of humid air containing low concentrations of atmospheric contaminants. Examples of atmospheric contaminants include but are not limited to organic vapors (e.g., fuel vapors), NOx and SOx; however, depending on the location of the CP filter, atmospheric contaminants may also include cleaning and degreasing solvents, engine exhaust, mercaptans and sulfides, etc. For example, a filter designed to treat 200 ft3/min of air may process greater than about 100,000,000 ft3 of air per year. Due to the large volume of air which passes through the CP filter, the exposure of airborne contaminants poses a significant degradation hazard to the overall service life of the filter. A CP filter's ability to remove traditional CW agents and TICs may be decreased upon contact with one or more airborne contaminants. The net effect of continuous or near continuous exposure of airborne contaminants to the CP filter media causes a degradation of the activated impregnated carbon media contained within the filter. Due to degradation resulting from continuous or near-continuous operation of the CP filter, the CP filter is often “overdesigned;” that is to say, the filter contains a greater than necessary volume of activated, impregnated carbon to take degradation into account.
While not meant to be limited thereby, it is believed there are two primary mechanisms by which airborne contaminants may adversely affect the ability of activated, impregnated carbon to provide chemical protection. First, a contaminant may react with the impregnants responsible for the removal of toxic chemicals via chemical reaction. Examples of airborne contaminants that may react with the impregnants may include but are not limited to acid gases, e.g., hydrochloric acid, nitric acid, chlorine gas, and sulfur dioxide, etc. Many of the chemical reactions promoted by activated, impregnated carbon are of the gas-solid type. For example, sulfur dioxide may be oxidized within the pores of carbon granules, leading to the formation of sulfur trioxide (SO3). Sulfur trioxide may subsequently react with one or more base metal impregnants, for example, copper ammonium carbonate. Such a reaction with copper ammonium carbonate may lead to the formation of copper sulfate (CuSO4). Copper sulfate is relatively ineffective in its ability to react with certain traditional CW agents, for example, hydrogen cyanide.
Secondly, low volatility airborne contaminants, such as, for example fuel vapors, may be physically adsorbed within the pores of activated, impregnated carbon, preventing access to impregnants located within the pore structure, minimizing the adsorption of persistent threat compounds (e.g., sarin, pesticides). Examples of airborne contaminants that may physically adsorb within the pores of carbon include oils, hydraulic fluids and insecticides. Examples of airborne contaminants that may polymerize within the pores of activated, impregnated carbon include acrolein, formaldehyde and ethylene glycol. In addition, exposure of a filter to moderate volatility airborne contaminants (e.g., fuel vapors) may adversely affect the ability of the filtration media to remove toxic chemicals, at least until sufficient time has elapsed for the adsorbed vapors to purge from the media. Furthermore, fuel vapors may degrade the performance of a CP filter by physically blocking the pores of activated, impregnated carbon. This physical blocking may prevent access of one or more of the CW agents and TICs to reactive impregnates located within the pores of the carbon granules. As a result of prolonged exposure to the ambient environment, the media in the collective protection filter may become contaminated. The contamination may reach a point where the filter may no longer be able to provide a threshold level of chemical protection against CW agents and TICs.
Activated, impregnated carbon may also be degraded by long term exposure to humid air. The impregnation complexes associated with activated, impregnated carbon are often times water soluble. Over time, said impregnants migrate from within the pores of the carbon granules to the external surfaces of the granules. Said migration leads to a loss of metal dispersion and subsequent degradation in filtration performance.
It is desired that collective protection filters be as resistant as possible to performance degradation resulting from exposure to humid air and atmospheric contaminants. In this manner, filters may remain in service for extended periods of time so that when needed, the filters may provide the necessary level of protection. In addition, increasing the durability of the filters may decrease costly replacement and disposal burdens.